1. Field of the Invention
The present invention relates to a light source driving apparatus for monitoring a part of the light emitted from a light source such as a semiconductor laser and controlling said semiconductor laser based on the result of said monitoring, thereby achieving APC control for maintaining a constant light quantity or modulation control for obtaining a desired exposure, and to a system such as an image recording apparatus utilizing said light source driving apparatus.
2. Related Background Art
A semiconductor laser light source tends to show unstable light output for example, because of temperature change. Particularly, when such semiconductor laser is employed as the light source of a scanning optical system requiring a predetermined output intensity such as in a laser beam printer, it is necessary to effect so-called APC (automatic power control) by splitting and monitoring a part of the emitted light and effecting feedback on the drive current to maintain a predetermined light output.
The light emitted from a semiconductor laser is generally a mixture of the laser oscillated light linearly polarized in an oscillating direction parallel to the active layer and the LED oscillated light randomly polarized in all the directions, and, since the proportion of the laser oscillated light increases rapidly when the drive current exceeds the threshold value of laser oscillation as shown in FIG. 8, the ratio of mutually orthogonal polarized components is variable depending on the magnitude of the drive current.
For this reason, when the conventional APC is performed by splitting a part of the light emitted from the semiconductor laser, the monitoring light beam has been obtained, as disclosed in the U.S. Pat. No. 4,844,584, in such a manner that so-called non-polarizing beam splitter S capable of splitting mutually orthogonal polarized components in same proportions is employed as shown in FIG. 9, and a photodetector D is located on a split optical path LR. An alternative method has been obtained in which only a specified polarized light component is taken out from the emitted light by, for example, a polarizing beam splitter, and then a monitoring light beam is obtained from said polarized component by an ordinary beam splitter, thereby maintaining a constant ratio between the monitoring light beam and the light output.
On the other hand, in the U.S. patent application Ser. No. 584,263 filed on Sep. 18, 1990, a method for modulating the semiconductor laser have been proposed in which the laser is capable of providing a desired exposure regardless of variation in temperature, without relying on the APC mentioned above. An outline of one embodiment of said proposed method is such that the semiconductor laser is driven with a waveform whose output of light gradually increases, a part of said light output is monitored by a photodetector, and the output of the laser is allowed to increase until it reaches a predetermined value corresponding to the desired exposure.
However, the non-polarizing beam splitter and the polarizing beam splitter mentioned above require extremely complex manufacturing steps for obtaining special characteristics, in comparison with those for the ordinary beam splitter (splitting two polarized components with different proportions). For example the splitting face of these beam splitters is generally composed of multi-layered optical thin films, which, requiring at least 7 to 8 layers, cannot easily provide uniform characteristics and involve a high production cost.
In the non-polarizing beam splitter, satisfactory characteristics may be obtained with a single-layered thin metal film, but the film thickness in such case has to be strictly controlled, for example, in a range of several nm to over 10 nm, and the formation of such thin film in uniform manner is extremely difficult.
Also, the above-explained structure employing the polarizing beam splitter requires another beam splitter, so that it not only elevates the production cost of the entire apparatus but also deteriorates the efficiency of utilization of the emitted light.
Furthermore, even if the intensity ratio of the emitted light and the monitoring light can be always maintained constant, the monitoring light becomes proportionally weaker if weak emitted light is desired, so that S/N ratio of the detection signal from the photodetector becomes low to hinder adequate feedback control.